Middle cerebral artery flow velocity during coronary surgery; influence of clinical variables

The flow velocity in the middle cerebral artery was measured continuously with a noninvasive transcranial Doppler in 18 patients during coronary artery surgery. Neurolept anesthesia and alpha-static acid-base management were employed. The flow velocity data were expressed as a percent of the awake level. During intubation, there was a transient flow velocity increase, which was related to a concomitant increase in mean arterial pressure (r = 0.67, p less than 0.01). Prior to cardiopulmonary bypass, flow velocity had decreased to 52.4% +/- 3.0% (mean +/- SEM). At the onset of cardiopulmonary bypass, flow velocity values were transiently doubled. Flow velocity then reached a stable level of 63% to 65% during hypothermia (25 degrees C to 30 degrees C). The increase from 52.4% to 63% to 65% was related to the reduction in hematocrit (r = -0.62, p less than 0.02). With rewarming, flow velocity increased to 101% +/- 5.2%. Flow velocity was found to correlate with temperature during cardiopulmonary bypass (median rs = 0.84, range 0.61 to 0.99, p less than 0.0001). No positive correlation was found between mean arterial pressure (MAP) and flow velocity during cardiopulmonary bypass. Although no direct metabolic measurements were performed, it is concluded that these findings are compatible with a maintained cerebral blood flow/metabolic coupling during cardiopulmonary bypass.